We have three different applications (or three different versions of same application) and each application should require different activation code on same computer. How we can accomplish this?
You just need to be sure that you protect each application with different project. Specifically, you need to be sure that Application signature and Program ID values are different (unique) for each of these applications. This means creating and handling three different projects for three applications.
Use correct project settings when generating activation codes for each of these three applications. If wrong project is used for generation of activation codes appropriate error will be displayed.